The present invention relates to a rechargeable battery that can use a negative electrode active material having a high volume expansion rate by receiving volume variation of an electrode assembly in a case. The rechargeable battery according to the present invention includes i) an electrode assembly including a positive electrode plate, a negative electrode plate, and a separator that are wound in a jelly roll shape to have a flat portion and curved portions disposed at both sides of the flat portion, ii) a case for receiving the electrode assembly, and iii) a cap assembly combined with the case to close and seal the case and that is electrically connected to the electrode assembly. When a direction that is perpendicular to a surface of the separator disposed in the flat portion is set to a first direction, the thickness of the flat portion measured along the first direction is smaller than the maximum thickness of the curved portion measured along the first direction.